Fibrodysplasia ossificans progressiva (FOP) is a hereditary disease whose cardinal symptoms are swelling with inflammation or pain (also known as “flare-up”), and heterotopic ossification (also known as “ectopic ossification”). In patients with typical fibrodysplasia ossificans progressiva (FOP patients), flare-ups appear from childhood or school age, and heterotopic ossification occurs as flare-ups disappear. During this period, fibrous tissue of the skeletal muscle, fascia, tendons and ligaments, etc., undergoes progressive and heterotopic ossification throughout the body, resulting in articular contracture, deformation or mobility impairment.
Ectopic ossification in FOP is characterized by sudden ectopic ossification elicited by destruction and regeneration of soft tissue such as muscle (Non-Patent Literature 1). The ectopic ossification seen in FOP patients has been reported to take place via the process of endochondral ossification (or endochondral ossification) (Non-Patent Literature 2). Endochondral ossification is a phenomenon in which cartilage forms first and is then replaced by bone.